This invention relates to fuel cells and, in particular, to systems for compressing planar fuel cell stacks. More specifically, this invention relates to mechanical systems for maintaining a compressive load on stacks of high temperature fuel cells.
A fuel cell is a device which directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions.
In general, a fuel cell, similar to a battery, has a negative (anode) electrode and a positive (cathode) electrode separated by an electrolyte which serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells must be stacked in series with an electrically conductive separator plate between each cell.
A conventional fuel cell stack typically has several hundred fuel cells in series. In order to work properly, intimate contact must be maintained between all cells in the stack. Adequate contact must exist during all stack operating conditions for the duration of the stack""s life. Factors to be considered in achieving this requirement include manufacturing tolerances of the cell components, non-uniform thermal expansion of the cell components during operation and long term consolidation of the cell components resulting in shrinkage of the stack.
Accordingly, a variety of requirements are placed on the system used to compress the fuel cell stack. The system must apply enough load to overcome the manufacturing tolerances early in life to bring the cell components into intimate contact. The load must also be great enough during operation to prevent the cells from delaminating due to the inevitable thermal gradients within the stack. At the same time, the compressive load should not be so great as to cause excessive shrinkage of the stack during its life as this places undue demands on auxiliary stack hardware and on the required follow up of the compression system itself. An additional requirement is that the system does not completely relax over time to insure that adequate stack pressure is maintained through the end of stack life.
Conventional fuel cell stack designs use one of a number of mechanisms for applying compressive load to the stack. U.S. Pat. No. 4,430,390 describes spring members which run within the manifolds of the fuel cell stack attaching to the end plates and forcing them toward each other. This design is not desirable for high temperature systems such as molten carbonate and solid oxide stacks because the spring members would need to be excessively large and be constructed of exotic, corrosion resistant materials to withstand the high temperature environment. U.S. Pat. No. 4,692,391 describes a design where the end plates are directly connected by rigid tensile members such as bolts or threaded rods. However, this system provides practically no load following capability to maintain stack compression as the stack shrinks.
U.S. Pat. No. 5,686,200 describes small, twisted wire or ribbon springs which may be used to apply load to individual cells within a stack. This design is inappropriate for large area fuel cells as the separator plates to which the springs are attached could not be constructed to be stiff enough to insure adequate load was delivered to the central area of the cells. U.S. Pat. No. 5,789,091 describes the use of continuous compression bands which are wrapped around the stack and placed in tension. Again this method suffers from inadequate follow-up for stacks with significant long term creep.
U.S. Pat. No. 5,009,968 describes a stack compression system which utilizes a thin end plate structure so as to minimize temperature gradients across its thickness thereby minimizing thermal distortions of the end plate. It also describes the use of a resilient pressure pad between the pressure plate and the end plate to minimize the effect of thermal distortions in the pressure plate on the end plate. This design suffers from excessive electrical resistance and non-uniform electrical current collection in such a thin end plate structure.
U.S. Pat. No. 6,413,665 describes one method of stack compression previously employed by the assignee of the subject application in which rigid tie bars are used to span the top end plate. Rigid tie rods are connected to the tie bars and to a mechanical linkage near the bottom of the stack. This linkage connects the tie rods to a spring assembly located under the bottom end plate and oriented at a nearly right angle to the tie rod. This design is complicated and expensive due to the mechanical components which make up the linkage.
Another method of stack compression previously employed by the assignee of the subject application utilizes pressurized bellows to apply compression to the stack. A series of large bellows are disposed in a vessel that houses the stack. The bellows are located between the compression and end plates of the stack at each end. The compression plates are tied together with rigid tie rods and the bellows are pressurized with nitrogen, thereby applying load to the stack. This system is less reliable than a mechanical system because even the smallest gas leak will cause the bellows to depressurize. Also, the bellows are expensive as they must be designed to withstand the high temperature environment inside the fuel cell vessel. A further limitation of this design is that an actively controlled gas delivery system must be provided to ensure stack compression load is maintained. Yet another drawback of this design is that any maintenance that must be performed on the bellows requires that the vessel be opened and the adjacent compression plate be removed.
It is an object of the present invention to overcome the above and other drawbacks of conventional fuel cell stack compression systems. It is another object of the invention to provide a compression system that is easy and relatively inexpensive to assemble and maintain. It is a further object of the invention to provide a fuel cell stack compression system that can prolong the operating life of the stack. It is an additional object of the present invention to provide a compression system for use in conjunction with a fuel cell stack housed within a vessel, whereby compressive force can be adjusted without requiring dismantling of the vessel. It is another object of the invention to provide a compression system that accommodates flexing of compression plates during operation of the fuel cell stack. It is yet another object of the present invention to provide a compression system for use with either a horizontally disposed or vertically disposed fuel cell stack.
The above and other objects are achieved by the present invention, which overcomes the disadvantages of conventional compression systems by providing a fuel cell stack compression system with compression assemblies which are at least partially outside the fuel cell stack vessel. The compression system of the present invention is used in conjunction with a fuel cell stack having first and second ends and enclosed within a vessel having first and second end walls facing the first and second stack ends. The compression system includes: a first compression plate inside the vessel at the first end of the stack; a second compression plate exterior to the vessel and abutting its second end wall and facing the second end of the stack; at least one member extending along a face of the stack, the member or members each having a first end connected to the first compression plate and a second end extending to the second end wall of the vessel; and at least one compression assembly, each compression assembly being coupled to a second end of a member, and a portion of the compression assembly being disposed exterior to the vessel.